nitrides conference report
A
side from the weather, the delegates that went to Glasgow for the ninth International Conference on Nitride Semiconductors received an incredibly warm welcome. Maybe this is not that surprising – after all, Scotland’s most populous conurbation has worked hard to get itself known as ‘The friendly city’. However, this is not always the case, and at times the city’s reputation can be tarnished by the fans of the two big football clubs – Celtic and Rangers – who will never ever see eye to eye. As expected, the delegates were on far better terms than these rival supporters, but that didn’t mean that there wasn’t a whiff of confrontation in the air at some of the sessions.
One topic that continues to court controversy and encourage researchers to vigorously defend their own position is that of the origin of droop, the mysterious phenomena that accounts for the decline in nitride LED efficiency when the current in this device is cranked up. Talks on this topic were very well attended, and while there seems to be a growing consensus of opinion that droop involves some form of Auger recombination (a non-radiative process involving either two electrons and one hole, or two holes and one electron), the debate is far from over.
In contrast to the last two ICNS meetings in Korea and the US, none of the leading LED manufacturers spoke about droop during the conference. This may be because these leading chipmakers feel that they now have a good enough understanding of the causes of droop and they cannot justify the funding of further research into its intricacies. However, that is probably only part of the reason – in the past few years the efficiencies of state-of-the-art white-emitting LEDs have shot passed those of compact fluorescent bulbs, and chipmakers are now focusing on improving the manufacturing process of these nitride-based devices, so that they can drive a revolution in general lighting. In their absence, academics made the running in the droop debate, sometimes aided by commercial modelling software.
Droop: the case for Auger Some of the most compelling evidence for Auger recombination as the primary cause of droop was given in a talk by Wolfgang Scheibenzuber from Fraunhofer IAF. He and his co-workers, including members of Nicolas Grandjean’s group from EPFL, Switzerland, have determined the extent of Auger recombination in nitride laser diodes, the cousins of nitride LEDs.
Scheibenzuber began his talk by describing a pair of equations that he and his colleagues have used to
August / September 2011
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